The Briton had an offer to face domestic rival Kell Brook in a bout, which his team had said would prove more financially rewarding.

But he will instead be a heavy underdog against 31-year-old Crawford, who has held world titles in three divisions.

A news conference is due to take place early next week to outline a venue, either in New York or Las Vegas. Both are cities in which Khan has boxed before - this time, though, he will go up against an American widely regarded as one of the sport's best pound-for-pound fighters.

BBC Sport boxing correspondent Mike Costello ranked Crawford as his third best pound-for-pound fighter at the start of 2019

Khan picks one of world's best

Omaha-born Crawford has held world titles at lightweight, super-lightweight and welterweight, and has already been installed as 1-10 favourite with some bookmakers.

More notably, he was an undisputed champion at super-lightweight in 2017 when he overcame Julius Indongo to hold all four world titles in the division.

He moved up to the 147lb welterweight class to take his current title from Australia's Jeff Horn in June, and his most recent stoppage win over Jose Benavidez Jr in October took his record to 25 knockouts from 34 wins.

Khan has faced disapproval in some quarters for his September points win over Samuel Vargas, where he was knocked down before recovering to take his record to 33 wins and four defeats.

And Crawford, who boxes from the tricky southpaw stance, represents a huge step up as Khan bids to become a world champion for a second time having held titles at super-lightweight between 2009 and 2011.

Crawford has held world titles in three weight divisions

Brook to switch to Smith?

Khan has been criticised for avoiding Brook but has always insisted he hoped to pursue world titles upon returning to the sport and this challenge - albeit fraught with difficulty - offers such a chance.

Negotiations between Khan and Brook had dragged on, with Khan insisting any deal included a rehydration clause that would have prevented his rival from gaining excessive weight following any weigh in.

Khan's move now leaves Brook, 32, without an obvious opponent and undoubtedly disappointed given both he and his team had agreed to demands having pursued the lucrative bout in recent years.

Some have suggested Khan believes a meeting with Brook will still be available to him after he faces Crawford, though Eddie Hearn - who promotes both fighters - has publicly said negotiations had reached a stage where it was "now or never".

Brook will need to look elsewhere. Perhaps tellingly, former light-middleweight world champion Liam Smith raised his name as a potential opponent when he switched to the same promotional team earlier in the week.